This invention relates to an elastic surface wave device of which the characteristics are improved by using an elastic surface wave absorber having a particular configuration.
Attempts have conventionally been made to improve the characteristics of an elastic surface wave device by using an elastic surface wave absorber. Such a surface wave device is shown, by way of example, in FIG. 1. In this Figure an interdigital type input transducer 2 and interdigital type output transducer 3 are provided at a predetermined interval on one surface 1 of a piezoelectric substrate. A surface wave propagation path is provided by that surface portion of the substrate which is provided between the transducers 2 and 3. On the extension of the surface wave propagation path, surface wave absorbers 4 and 5 of rectangular configuration are provided one at each end portion of the piezoelectric substrate such that their side edges intersect substantially orthogonal to the surface wave propagation path. The interdigital electrodes 2a and 2b in the input transducer 2 have common connection electrodes 2a-1 and 2b-1, respectively, across a surface wave signal source 6, and the respective common connection electrodes 2a-1 and 2b-1 have a plate-like portion as shown in FIG. 1 except for the overlapped portions of the digit electrodes. As a result, it is possible to prevent unwanted surface waves from being reflected on those digit electrode sections except for the overlapped portions of the digit electrodes. The interdigital electrodes 3a and 3b of the output transducer 3 have common connections 3a-1 and 3b-1, respectively, and are connected to, for example, a load 7.
When a surface wave signal is supplied from a surface wave signal source 6 to the transducer 2 on the conventional surface wave device, a normal surface wave A is transmitted from the input transducer 2 toward the output transducer 3 as shown in FIG. 1 and there are reflected surface waves B to I which reach the output transducer 3 through various paths from the input transducer 2. That is, the reflected surface wave B reaches the output transducer 3 after reflected on the rear end of the input transducer 2, the surface wave C is reflected several times between the input and output transducers 2 and 3, a surface wave D is incident upon the output transducer after reflected on the front edge of the absorber 5 which is disposed behind the output transducer 3, a surface wave E is incident upon the output transducer 3 after reflected on the front edge of the absorber 4 disposed behind the input transducer 2, a surface wave F is incident onto the output transducer 3 after reflected on the rear edge of the absorber 5 on the output side, a surface wave G is incident onto the output transducer after reflected on the rear end of the absorber 4 on the input side, a surface wave H is reflected to the output transducer 3 after reflected on that substrate 1 end at the output side of the substrate 1, and a surface wave I is incident onto the output transducer 3 after reflected on the substrate 1 end on the input side of the substrate 1.
The reflected wave C can be reduced by adjusting the value of the load 7 and the reflected waves D to I be reduced by properly selecting the material and dimension of the absorbers 4 and 5. The suitable choice of the shape of the end surface of the piezoelectric substrate 1 reduces the extent to which the surface waves are reflected at the respective locations, thereby preventing degradation of the characteristics of the surface wave device. However, the surface wave B can not be reduced by any of the above-mentioned methods and it is difficult to prevent degradation of the characteristics of the surface wave device due to the presence of the reflected wave B.
It is accordingly the object of this invention to provide an excellent elastic surface wave device which can alleviate any influence resulting from surface waves reflected on the ends of transducers in particular.